The present invention relates to gas oil compositions, more specifically to gas oil compositions for a summer or winter season, suitable for both diesel combustion and homogeneous charge compression ignition combustion.
Diesel combustion is referred to as combustion wherein ignition occurs (premixed combustion) when fuel injected to an engine combustion chamber evaporates, mixes with air and becomes a premixed gas with an appropriate fuel/oil ratio, and the gas undergoes an appropriate temperature condition. It is often the case that whether this ignition is good or poor is determined by evaluating the evaporation characteristics resulting from the distillation characteristics and cetane number indicating the self-ignition properties of fuel. If a higher output is required in the diesel combustion (high load conditions), it is necessary to continue the injection of fuel even after the self-ignition occurs. In this case, the fuel must be combusted while the injected fuel is diffused in air atmosphere using fluidized air in the interior of the engine combustion chamber (diffusive combustion). Therefore, what are demanded for fuel characteristics are those to assist premixed combustion and diffusive combustion.
There is a combustion mode referred to as homogeneous charge compression ignition combustion deriving from these diesel combustion modes, and recently this combustion mode has brought attention because of the low emission properties and excellent fuel efficient properties. This combustion mode is different from the foregoing diesel combustion in that the whole combustion process of the former is premixed combustion and thus is not involved with diffusive combustion. However, as described above, ignition undesirably occurs due to the self-ignition properties of fuel and thus it is regarded as difficult to control ignition under high load conditions in particular. Therefore, there are many engines employing a combustion mode wherein homogenous charge compression ignition combustion is carried out only under low and middle load conditions and switched to a normal diesel combustion under high load conditions. Therefore, it can be concluded that what are demanded for fuel characteristics are those to have both a factor to assist homogenous charge compression ignition combustion in low load conditions and a factor to assist diesel combustion under high load conditions.
In general, a gas oil composition is produced by blending one or more types of base oils produced by subjecting a straight gas oil or straight kerosene, produced by atmospheric distillation of crude oil to hydrorefining or hydrodesulfurization. In particular, it is often the case that the blend ratio of the foregoing kerosene base oil and gas oil base oil is adjusted in order to ensure the cold flowability during a winter season. If necessary, the base oils are blended with additives such as cetane number improvers, detergents and cold flow improvers (see, for example, Non-Patent Document No. 1 below).
With regard to fuel for the above-described homogenous charge compression ignition combustion, Patent Document No. 1 discloses a diesel gas oil composition characterized in that it contains a relatively light catalytic cracked gas oil and is low in cetane number and high in density and aromatic content. This document describes that this composition can have both excellent low-temperature properties and low NOx and low PM properties in a homogenous charge compression ignition combustion application. However, it is easily anticipated that the aromatic content of the composition will be extremely large, leading to an increase in discharge of unburnt fuel. Further, as described above, it is currently often the case that homogenous charge compression ignition combustion is used in parallel with the conventional diesel combustion. It is thus apparent that the fuel of this document with a low cetane number, a high density and a high aromatic content is not suitable at all for homogenous charge compression ignition combustion. Further, it is also easily anticipated that soot or deposit will adhere to injection nozzles or EGR (exhaust gas recycle) control valves, due to the high aromatic content. Therefore, the diesel gas oil composition of this document fundamentally fails to be an environment friendly fuel. Similarly, Patent Document Nos. 2, 3 and 4 disclose that fuel compositions with distillation characteristics defined by functioning are effective for homogenous charge compression ignition combustion. However, as described above, distillation characteristics is not chemically involved with a factor to control the self-ignition properties of fuel, and in particular on the assumption of an improvement in homogenous charge compression ignition combustion of type wherein fuel is injected in an earlier stage, as proposed by the present invention, distillation characteristics are still less effective. An index which is defined by the temperature every certain distillate volume such as T90 but not by fraction volume can be a rough standard to learn the identity of fuel but does not make sense since it is not an absolute quantitative definition. Further, these fuel compositions are kept down in cetane number but are then likely to be reduced in the content of saturated hydrocarbon compounds and thus can be regarded as fuels which can not control ignition any time. Therefore, it is apparent that the characteristic definitions proposed by these documents can not be regarded as fuel characteristics that can control self-ignition, and then it is presumable that no environment friendly fuel has not accomplished yet.
Further, the environment friendly fuel is necessarily optimized in fuel characteristics every season in view of the environment where it is used. Fuel with excessively lightened distillation characteristics may often cause the seizure of injection pumps, cavitation damages and problems in high-temperature startability.
That is, it is very difficult to design a high-quality fuel that can achieve at a high level both the requirements sought for a gas oil composition having both an excellent practical performance under conditions in a summer or winter season and environment friendly properties that can be applied to homogenous charge compression ignition combustion, and there exists no example or finding on the basis of studies of such a fuel satisfying various properties required for fuel other than the foregoing sufficiently and a practical process for producing the fuel.    (1) Patent Document No. 1: Japanese Patent Laid-Open Publication No. 2006-28493    (2) Patent Document No. 2: Japanese Patent Laid-Open Publication No. 2005-343917    (3) Patent Document No. 3: Japanese Patent Laid-Open Publication No. 2005-343918    (4) Patent Document No. 4: Japanese Patent Laid-Open Publication No. 2005-343919    (5) Non-patent document No. 1: Konishi Seiichi, “Nenryo Kogaku Gairon”, Shokabo Publishing Co., Ltd., March, 1991, pages 136 to 144